UPS static bypass switch — thyristor switching and make-before-break transfer

The guide on DRUPS covers why the short-circuit capacity of a rotary UPS differs from a static UPS. This guide covers a core component of precisely that static, double-conversion UPS: the static bypass switch. This is one of the most critical — and most underestimated — components of a UPS installation, since it safeguards continuity the moment the inverter itself can no longer supply the load.

What the static bypass switch does

In a double-conversion UPS ("online" UPS), the load is normally fed via the inverter: rectifier → battery/DC bus → inverter → load. The static bypass switch is a parallel path, built from thyristors (SCRs, anti-parallel per phase), that connects the load directly to the bypass mains (usually the same supply as the UPS input). As long as the inverter operates normally, this path is inactive (the thyristors do not conduct). Upon an inverter fault or overload, the control system fires the thyristors, transferring the load to the bypass mains almost instantaneously.

Why thyristors and not a mechanical relay

A mechanical relay or contactor has a switching time on the order of tens of milliseconds — too slow to transfer a sensitive IT load without a voltage interruption. Thyristors switch on within a fraction of a mains cycle, typically within a quarter cycle (~4-5 ms at 50 Hz), and moreover only at a current zero crossing, so no abrupt current or voltage disturbance occurs. This makes the transfer essentially seamless for the load.

Synchronization requirement: why it isn't always seamless

A seamless, "make-before-break" transfer is only possible if the inverter output and the bypass mains are synchronized in phase, frequency and amplitude at the moment of transfer. The UPS control system continuously monitors this and normally keeps the inverter synchronized with the bypass mains, precisely to enable an instantaneous transfer when needed. However, if the bypass mains falls outside the synchronization window (for example a different frequency or an excessive phase deviation, as can occur with a generator supply lacking proper synchronization), a seamless transfer is not possible, and either a brief interruption ("break-before-make") follows or the transfer is refused until synchronization is restored.

Automatic transfer scenarios

The static bypass switches automatically upon:

Inverter overload above the short-term overload capacity (for example a large inrush current from the load or a downstream short-circuit, where the bypass temporarily delivers the fault current the inverter itself cannot supply — see also the considerations in the guide on main fuse selectivity for the underlying rationale of sufficient available short-circuit current to trip a protective device selectively).
Inverter fault or shutdown (for example overheating, internal fault).
Manual transfer by the operator via the control panel, for maintenance on the inverter while the load remains fed.

Distinction from the manual (mechanical) maintenance bypass

Besides the static bypass switch, nearly every UPS installation also has a manual maintenance bypass switch: a mechanical, lockable switch that removes the entire UPS unit — including the static bypass itself — completely from the power path, so that maintenance on the UPS unit itself (including the static bypass circuit) can be carried out safely without interrupting the load. The static bypass protects against faults during normal operation; the manual maintenance bypass enables maintenance on the entire UPS. Confusing the two — for example assuming the static bypass is sufficient to safely work on the UPS unit — is a known pitfall, since the static bypass itself remains energized during maintenance on the inverter.

Note: work on the UPS unit itself (not just the batteries) must only be carried out after the manual maintenance bypass has been engaged and the UPS unit has been isolated per the applicable LOTO procedure — not on the basis of the static bypass alone.

Practical relevance

When assessing the availability of a UPS installation, it is important to verify that the bypass mains has sufficient short-circuit capacity and voltage quality to actually supply the load during a bypass transfer — a bypass switch that transfers to a weak or unsuitable supply does not actually solve the availability problem. In addition, phase selectivity between the protection of the bypass path and that of the load must be coordinated, so that a downstream fault during bypass operation is isolated correctly.

Common mistakes

Assuming the static bypass switches mechanically — the thyristor switching is electronic and has a switching time of milliseconds, not tens of milliseconds as with a relay.
Confusing the static bypass with the manual maintenance bypass — only the latter enables safe maintenance on the UPS unit itself.
Assuming a bypass transfer is always seamless — without synchronization between the inverter and the bypass mains, this is not the case.
Not checking the bypass mains for sufficient short-circuit capacity, so that a transfer to bypass during a fault still fails to adequately supply the load.